This invention relates generally to surface acoustic wave devices and more particularly to techniques for adjusting the surface acoustic wave velocity and, thus the frequency characteristics of a surface acoustic wave device.
As it is known in the art, surface acoustic wave devices such as resonators, delay lines, filters, and pressure transducers are used in a variety of applications. Generally, a surface acoustic wave device (SAW) comprises a pair of transducers, with each transducer having a set of conductive members which is disposed on or recessed within an upper portion of a surface which supports surface acoustic wave propagation. As surface acoustic wave devices find new applications, the requirements for precision in the frequency characteristics of the surface acoustic wave device increase. Accordingly, in many applications, it is now desired to have the center frequency of the device within .+-.1 ppm of the designed for center frequency. Many factors contribute to deviations from the designed for center frequency of a SAW device including the fabrication techniques presently used to manufacture SAW devices. Typically, with present techniques, the after fabricated SAW device has an actual center frequency within about .+-.100 ppm of the designed for center frequency. Accordingly, the frequency characteristic of the fabricated devices must be modified either upwards or downwards in frequency to meet the designed for center frequency.
Several techniques are commonly employed in the art to change the frequency characteristics of a SAW device. One technique known as air-baking involves exposing the SAW device to air disposed at an elevated temperature for a limited period of time to produce an upshift in the center frequency of the device. The utility of air-baking is relatively limited, however, since air-baking has not proven to be a reproducible technique, and furthermore, the amount of frequency shift obtained during the air-baking process is extremely limited particularly at frequencies below 500 MHz. A second technique involves etching techniques such as reactive ion etching. The reactive ion etching techniques involves sophisticated equipment, in which the SAW device is exposed to fluorine ions produced by a r.f. discharge. The fluorine ions selectively etch the surface wave propagation surface. The result of reactive ion etching is to trim down the center frequency of the SAW device. With reactive ion etching, frequency adjustment as much as -500 ppm may be obtained. Reactive ion etching, however, involves the use of relatively expensive and sophisticated equipment and, furthermore, the technique may involve relatively long etching times for devices in which a large frequency adjustment is necessary.
A second technique known in the art is set forth in U.S. Pat. No. 4,234,960 by David J. White et al and in papers entitled "Fine Tuning of Narrow-Band SAW Devices using Dielectric Overlay" 1977 Ultrasonic Symposium Proceeding, IEEE, pgs. 659-663 by Helmick et al. and "Observation of Aging and Temperature Affects on Dielectric Coated Saw Device", 1978 Ultrasonics Symposium Proceedings, IEEE, pp. 580-585 by Helmick et al. These papers and patent describe a technique in which a dielectric coating is provided on the surface wave propagation surface and in contact with the electrodes forming the interdigitated transducers, with the amount of frequency shift selected by controlling the thickness of the deposited coating. While the described technique produces frequency variations, these frequency variations come at the expense of a relatively large increase in the insertion loss of the device generally in the order of 1 db to 2 db, as well as, a relatively large increase in the so-called "turnover temperature" of the piezoelectric material which supports the surface acoustic wave propagation.
As it is known, some materials which are commonly employed to support surface wave propagation such as ST-cut and rotated ST-cuts of quartz, exhibit a parabolic surface wave velocity variation as a function of temperature. The maxima point of this parabolic variation is referred to as the turnover temperature. In many applications, the SAW device is designed to operate close to this temperature, particularly when the frequency stability of the SAW device is of critical importance. Large unpredictable variations in the turnover temperature would place the device out of specification for these applications, since the cut of the substrate material is specified for its particular temperature dependent characteristic. Accordingly, the large shifts in the turnover temperature described in the above references could make this technique impractical for use in many SAW device applications. Therefore, it is desirable to provide a technique which will have accurate and reproducible adjustments in frequency characteristics without significantly affecting other properties of the SAW device.